Manufacture of oleum



' Feb. 5, 1946. B. M. CARTER MANUFACTURE OF OLEUM Filed Jan. 26, 1942` lNvENToR erafa (brief ATTORNEY- Patented Feb. 5, 1946 Bernard M. carter, Montclair, N. J., assignmto General Chemical Company, New York, N. Y., a' corporation oi' New York A Application January 26, 1942, Serial No. 428,223

3 Claims.

This invention relates to improvements in processes for manufacture of oleum.

According to priorpractice, low and intermediate strength oleum, e. g. up to say 40%, on the one hand and high concentration oleum such as 60% strength on the other hand have been made by two radically dlering procedures. For example in production of about 40% or weaker oleum, the SO: gas exiting an -SO2 catalytic oxidation plant at temperature of 42E-875 F. is ordinarily cooled down to about 250 F. or less, heat thus extracted from the Sm-gas is removed from the system, and

the cooled SO: gas is introduced into the bottom of a countercurrent absorption tower operated in such a way that the so-called "make drawn off the liquor circulating circuit of the absorption tower is oleum of the desired strength. i

This inethod for making intermediate or low strength oleum is subject to substantial operating y disadvantages. First, it is necessary to cool down the contact plant S03 gas from temperatures usually of the order o! G50-850 F. to about 250 F. when making 40% oleum) or less before introducing the S03 gas into the rst absorption tower. Incoming S03 gas is cooled to the extent 'indicated because if the S03 gas enters the bottom oi -the rst absorption tower at too high temperatures it vis not possible to form in the tower the oleum product of the sought-for strength. Heat thus taken out of the gas is removed from the system and serves no useful purpose therein. More importantly, because of the fairly low S03 content by volume of contact plant gases of most commercial installations, such SO: concentration being about 8.5% or less, and also on account of the rapid rise in vapor pressure of oleum at higher concentrations, high strength oleum cannot be made by the direct absorption process. Further, when working with some commercially available low strength SOa gases, even when making oleum of strength not higher than about 40% by direct absorption, unless a commercially uneconomical number of absorption towers Vis used, it is not possible to operate the S03 absorption system so that the system is so-called balanced, i. e. operated so that all of the incoming S03- whether introduced as SO: gas or as S03 constituent of any liquid diluent used-is converted to oleum ofv the desired strength. and no SO: is taken out of the system as intermediate strength oleumor as sulfuric acid. It will be understood that in an unbalanced systemproducing say 40% oleum as the desired product, operation is such that a portion of the incoming SO: is drawn out of the system as either sulfuric acid from the nal sulfuric acid production tower, or as relatively low strength say 2,0% oleum from the intermediate low strength oleum production tower.

The principal object of this invention is to provide procedure by practice of which high strength Y the objects of `the invention.

oleum may be made by the direct absorption method even from low concentration S03v gas. Another object constitutes provision of a process according to which the S03 concentration of commercially available S03 gas streams may be sufficientiy enriched so that such gas streams may be used for production of high strength oleum by direct absorption.- Afurther important object is i provision oi a process by which it is alternatively possible to make low or intermediate strength oleum by direct absorption in apparatus in which the absorption towers and associated liquor circulation circuits` are from to 55% smaller` than corresponding apparatus needed in accordance with the prior art practice to produce oleum oi low or intermediate concentration. The invention aims Ato employ the heat which, in the prior art practice, is'removed from the system at the point between the, ga's exit of a contact plant and the gas inlet of the S03` absorption and oleum production system, and to utilize this heat in such a manner as to p'rovide for accomplishment of The invention, its objects and the aorded ad- 'vantages will appear from the following description taken in connection with the accompanying diagrammatic drawings, showing for illustrative purposes, in Fig. 1 apparatus suitable for use in production of 60% oleum, and in Fig. 2 apparatus for manufacture of 40% oleum.

' Referring to the drawing, the oleum stripping or S03 desorption towers I0 and il are constructed substantially the same as known sulfuric acid or oleum absorption towers and comprise e steel shells lined with acid-proof brick and acidproof checkerwork or other packing suitably arranged to permit high capacity liquid ow and low resistance to gasflow. Each stripper is arranged for ,direct gas and liquid. contact, and while 40 cocurrent stripping may be employed, countercurrent ilow relation of 'gas and liquid is preferred. The strippers are provided at the topv with a gas outlet and a liquor inlet including a' suitable liquor distributor designed to effect thorough dissemination of liquor particles throughout the desorption zone. At the bottom of the stripping towers, beneath the grillework supporting the packing, is a liquor outlet and a gas main such as Il or I5 connecting the systems illustrated'with the as outlet of the last converter or of the last heat exchanger of a contact plant operated to catalytically oxidize SOzto S03.

The designand construction of oleum production towers J8, t9 and 20 (Fig. 1) and 2l, 22 (Fig, 2) are usually the same as that of oleum production towers known in the art, and are ordinarily brick-lined steel shells illled with brick checker# work or other adequate packing. Each tower includes at the top a. gas outlet and a liquor inlet and distributing head, and at the'bottom a 28, 29 and each. of the oleum and sulfuric acid production towers is provided with its own liquor circulating circuit including the respective coolers mentioned along with suitable pipe connections indicated diagrammatically on the drawing, together with pumps and control valves, etc., not shown, all arranged to maintain circulation over the v arious towers of suitable quantities of oleum or sulfuric acid kept at temperatures required to accomplish the desired S03 absorption in any particular oleum or sulfuric acid `production tower.

Incoming S03 gas from a contact plant enters thru conduits Il or l5, passes in series thru the several towers, tail gas containing substantially no S03 leaves the tops of sulfuric acid production towers 38 and 39 and is discharged into the plant stack. Gas. movement thru the systems is indicated on the drawing by dotted lines. The diluent used, either water or sulfuric acid of suitable HzS04 strength, is fed into the circulating circuits of sulfuric acid production towers 38- and 39 thru inlets 5,0. and 5I. Liquor ow lthru the systems, indicated on the drawing by 4solid lines, is generally countercurrent to gas .ow. The so-called make of oleum production towers i8 and 2l, drawn onl thru lines 55 and 56, constitutes the products of the process.

To take best advantage of thebenets of the invention, the S03 gas is introduced into a stripping or desorption tower at a temperature as high as practicable. As is known in catalytic SO2 oxidation practice, temperatures of the exit portion of the 20 and 22 of the drawing. Since it is desired to gas of the last converter in a catalytic S02 oxida- .i

tion plant are roughly 825-875 F., the upper limit of such 'temperature being selected to effect maximum conversion of S02 to S03 while avoiding dissociation of S03 to SO: and oxygen.

Some S02 contact plants include a h'eat exchanger thru which the S03 exit of the last converter is immediately passed, Vsuch :heat eX- changer being used to aid in preheating incoming S02 gas up to initial conversion temperature. In instances wherefsuch a heat exchanger is employed, the temperature of the S03 exit gas of this heat exchanger is .ordinarily Amore' than 200 F. less than the temperature of the gas as discharged from the last converter. Hence depending upon the particular type of contact plant used, the temperature of the S03 `gas leaving the S03 converter plant may vary from about 425 to about 875 F. In the present process, S03 gas is fed into the bottom of the stripping tower at a temperature as high as feasible, the upper temperature limit being that at which S03 dissociates to S02 and oxygen, the lowertemperature linut of the S03 .gas being not less than aftemperature which is high enoughy to effect, in the stripping tower, desorption or release of S03 from the particular oleum fed, into'the top of the stripping tower at the vparticular' desorption conditions under which such tower is being operated. However, it is preferred to take the hot SOa gas directly from the'exit `pipe ofthe last convertervof the contact plant, although there is no objection to using the'SOa gas exiting the l l strength oleum being heat exchanger immediately following the last converter, if the S03 conversion system -as a 'whole happens to be an arrangement which includes a heat exchanger immediately associated with the outlet of each converter in the system.

.Further in the better practice of the invention;

the temperature of the 'incoming S03 gas should be not less than 150 F. and preferably not less than 250 F. above the temperature of the lower stripped at the point of iirst contact of incoming S03 gas with oleum being stripped, e. g. at tower illustrated.

According to this invention, there is provided an S03 stripping or'desorption tower, the function of which is to enrich the S03 content of the absorbing system gas stream and to eiect such enrichment by directly contacting in the stripper relatively low strength oleuml and lS03 gas under S03 desorption conditions such as to drive out or release S03 from such low strength oleum and to form a resultant composite S03` gas leaving the stripper having an S03 content higher than the S03 content of the `S03 gas entering the stripper. y Strength of the oleum fed into the top of the stripper may vary from a concentration ora few percent up -to something less than that of the ultimate high strength oleum product of the process. Ordinarily, the oleum supplied to the stripper is of concentration about half that ofthe product and isvpreferably a ortion of the oleum make of the oleum tower in' which the major total SO: is absorbed, e. g. towers carry into the stripper as much heat as possible,

intermediatestrength oleum is charged into the stripper without cooling and approximately at the maximum temperature of production.

In the stripping step the heat which, in accordance with this invention, is utilized to effect S03 enrichment o! the main S03 gas stream is the sensible heat contained in the gas entering the stripper over and above the sensible heat contained Ain the gas stream as introduced into the bottom of the' ilrst absorbing tower. To eiect stripping of S03 from any particular intermediate strength oleum, desorbing operation conditions should be such that (a) temperature of oleum leaving the stripper is in excess of the temperature at which oleum of the strength being fed to the desorbing operation is in equilibrium with 4the S03 concentration of the particular hot S03l centration and suitablyy high temperature desorption of S03 from any particular intermediate strength oleum used trol or regulation of at least tors such as quantity of oleum fed into the Zone, temperature 'of such oleum, and the rate of flow of the S03 gas 'thru the zone. Practice of the stripping operation is such that the gas stream leaving the stripper has an S03 content which is from 2 to 18V or more volume percent greater than.'

the volume percent of S0: in the gas entering the stripper.- Extentcfstripper exit gas SOaen-4 richment depends upon factors such as *strength the bottom of stripping iseffected by suitable con-w A one of operating factower I8, introduction of about 59.5% oleum of the oleum ied into the stripper, temperature of the S03 gas entering the stripper, rate of gas ow thru the stripper, and temperature and quantity of oleum fed into the stripper. With respect to temperature reduction of the incoming S03 first absorbing tower.

In the use of an absorption tower, such as tower I8 Fig. 1 of the drawing, the tower is operated volume in excess (preferably at S03 concentration i oleum at the point of last contact of S03 gas and The maior advantages afforded by the invention lie in' production of relatively high strength I In a commercial size. installationyequilibrium conditions between gas' oleum, e. g. 40% and above. importan it is practical to cool the circulating liquors in commercially economicalcoolers without resorting to expensive refrigeration. In practice, it is ure less than about 10 F. above the freezing point of the oleum .at such point. available cooling equipment which does not include refrigeration. it is net practical particularly in summer to attempt to cool the liquors at any point in the S03 absorbing systemto less than about 75 F. Hence, having in view the relatively ilxed lowerA temperature limits, i. e. say 75 F. in a commercially practical absorption system or a temperature at least about 10 F. higher than the freezing point of the oleum, together with Sharp rise, in the vapor pressure of oleum at higher strengths, my invention Ipossesses the substantial operating advantage of facilitating formation of an SO.; gas stream of strength high enough to providefor absorption temperature requirements and to provide a high strength S03 gas in the system from .the gas inlet of the iirst absorber to the gas outlet of the last oleum pro- A ducing absorber. l f

The S0: concentration of available contact plantconverter gases ordinarily ranges from about 6.5% to about 10.5%' by volume although Y in some cases where smelter gases constitute the tration oleums, of enriching operation of catalytic processes using brlmstone burners `to provide SO2, S03 concenof converted gases may 11%. This invention is directed preferably to the utilization of these commercially available 5-11 S03gases. However, it will be noted that current commercial catalytic SOnoxidation practice is such that the S03 concentration of converter exit gases is too low to permit use of such gases for production of high strength oleum by direct absorption. The invention oil'sets this diiliculty. To illustrate, assuming it is desired to produce 60% oleum as indicated in Fig. 1 oi the drawing, and alsoassuming it is impractical in economical cooling equipment to cool the oleum circulating over tower I8 to such an extent that the temper. ature of the oleum entering the tower at B0 is much less than about F., from previous discussion of equilibrium conditions in the tower it will be understood that in order to eiiect appre.- ciable absorption of SOarin the tower the S0; content of the gas entering the bottom of the tower I8 thru line 6I should be something in excess of about 14% by volume. By practice of this present invention, the S03 concentration of an SO2 converter exit gas may be increased readily in the stripper stripper exit gas S0: content is well above the needed 14%.

The invention affords the primary advantage. especially in the manufacture of the high strength the SO.: content of the main Furthermore, in

rise as high as- Ill to such an extent that the quires use of expensive i 344.6 pounds perature of 205 I8 thru pipe 14 S03 gas stream in the system so that the S03 concentration is high enough to make possible the use of such gas stream for reproduction of high strength oleum by simple direct absorption procedure. Moreover, such S03 enrichment is eftected-by utilization of the sensible heat of converter plant exit gases which sensible heat, according to. all prior methods known to me for making oleum. Ais taken out of the gas stream and removed from the entire system at a' point between the converted outlet and the inlet of the absorption tower.

In the prior practice for manufacture of relatively high strength oleum, e. g. above 40%, low or intermediate strength oleum is heated in a vessel equipped with steam coils to vaporize off substantially 100% S03 which is thereafter absorbed in medium strength oleum to produce the desired high strength product. This practice reboiling vessels and heating coils, .and consumption of extraneous heat (steam) to boil ofi the S03, and since the procedure is batch-wise, it is often necessary to repeat the operation several times before obtaining oleum of the desired high strength. Another factor involved in such prior herent diiculty of inducing adequate now of the substantially 100% S03 thru the oleum in which the S03 is to be absorbed, and the art has been compelled to resort to various inconvenient and none too eiIective means to bring about the required ow.

In the following examples, rates of gas ilow aregiven in terms of cubic feet per minute, quantities of liquors are given in pounds per minute, and liquor production of each of the absorbing towers is in pounds per minute. Further, rate of liquor-feed to th several towers in the systems described may vary from about 1000 to about 3200 pounds per minute, .the exact quantity being determined by the temperature control desired in such towers.

Example 1 is illustrative of manufacture of 60% oleum in apparatus such as'disclosed in Fig. 1 of the drawing:

` Example 1 `57:1 4pounds (25a cubic feet) of volume, isfed into the bottom Gas containing S03, 8.6% S03 by of stripper I8 thru pipe I4 about 500 F. and at a rate of minute. is obtained by roasting is oxidized at temperature o to S03 in a heat exchangerpis associated with each converter. Hence, the 500 F. temperature -of the gas in line I4 is substantially less than the temperature of converter. The 30% oleum production tower 20 is operated so that' the 30% oleum running o ut thr .pipe 10 is at a temperature a little above 160 F. of 30% oleum are drawn out from at temperature of tercurrent Vflow of oleum such that a oleum (283.8 pounds) at tem- F. is discharged from stripper by means of which such partially stripped oleum is returned to cooler and reintroduced inte the circulating system of the 30% oleum production tower 20. In stripper III, 60.8

pounds (272 cubic feet) of S03 are vaporized out of the 30% oleum fed in thru pipe 12 and the S03 gas leaying stripperIIl thru pipe 6Iat,tein

` strength is fed into the lit peratue of about 165? at concentration of 16.2%

( 530 cubic feet) of S03 by volume.

49.8 pounds per minute of the oleum of tower I9 lare run thru pipe 28 of tower are regulated so that make" of 16 into cooler oleum of about 59.6%v top of tower I8 thru pipe at a temperature of 80 F. Contactvof gas and liquor in tower I8 is such that 12.4 pounds (56.6 cubic feet) of S03 are absorbed, 60% Ioleum product formation amounting to 62.2 pounds per minute. The gas leaving the top of the tower I8 thru line 11, at temperature -of about F., contains 106 pounds (474.4 cubic feet)I of S03 and has an S03 concentration of 14.7% by volume.

35.4 pounds of make oi 30% oleum from tower 20 are run thru pipe 1I into cooler 29 for dilution in tower I9 circulating system in which liquor circulation and cooling are such that oleum of about 49.4%v strength is fed into the top of tower I9 thru pipe 19 at temperature of 105 F. 14.4 pounds (64.4 cubic feet) of S03 are absorbed and 49.8 pounds of 50% oleum are produced. A 13% by volume S03 gas, attemperature o about F. andl containing 9l.'1'poundsv (410 cubic feet) of S03, leaves tower I9 thru pipe 80 and enters the bottom of 30% oleum production tower 20. 283.8 pounds of 15% oleum from stripper I0 and 24 pounds of 99.3% sulfuric acid from tower 38 are introduced thru pipes 14 and 82 into cooler 80. Rate of cooling and of liquor circulation in the circulating system oi tower 20 are regulated so that oleum of 'strength of about 27% temperature of F. is fed 20 thru pipe 83. Gas and liquor contacting are such that '12.2 pounds (322.8 cubic feet) `of S03 are absorbed, and 30% oleum production amounts f 50 3000 cubic feet per' the S03 gas discharged from the .last

to 380 pounds. The gas stream passing thru pipe 66 into' the bottom of sulfuric acid production tower 88 is at temperature of about 115 contains 19.5 pounds and has an S03 concentration of 3.1% by volume.

The liquor cooling and circulating system of sulfuric acid tower 38 is operated to produce 99.3% strength sulfuric acid, and when so proceedlng 4.52 pounds per minute of diluting water are fed thru pipe 60 into the circulating system of tower 38. All of the 19.5 pounds of S03 entering thru .pipe 88 is absorbed, and the stack tail vgasis free of S03. Hence, in. this example, the system is balanced, i. e. all of the S03 is converted to oleum of the desired strength, and no S03 is taken out of the system as sulfuric acid or as oleum of any intermediate strength.

Example 2 This example illustrates production of 60% oleum in apparatus similar'to that disclosed in Fig. 1 of the drawing except that a single 35% oleumk production tower is used in place of the 30% and 50% production towersof the drawing. Use of a. single intermediate strength oleum tower is` made possible to a fairly strong incoming S03 and to a greater extent by the high S03 concentration obtainable in the S03 desorption tower.

Gas containing '71.8 pounds (321 cubic feet) of S03, 10.7% S03 by volume is fed into the bottom of a stripper at temperaturepf about '100 F. and at a rate of' 3000 cubic feet per minute. This S03 gas is obtained by burning brimstone to produce an SO; gas which is oxidized to S03 in a contact plant.

pounds v I8. Liquor cooling and circulationv and at a into the top of tower F. and (87.2 cubic feet) of $03,.

A 35% oleum production tower is operated so that the 35% oleum running out of the base thereof is at temperature a little above 140 F. 470 pounds of the 35% oleum make are fed at temperature of 140 F. into the top of the stripper in which countercurrent ow of oleum and incoming S03 gas is such that a 15% oleum (359.4

directly into the bottom of the 40% oleum yproduction tower. However, in most commercial pounds) at temperature of 210. F. is discharged centration being 23.3% by volume.

47.7 pounds per minute of the "make" of '35% oleum are run into the cooler of the 60% oleum production tower. Liquor cooling and circulation are regulated so that oleum of about 59.2% strength is fed into the top of the 60% oleum production tower at temperature of 91 F. Contact of gas and liquor in the 60% oleum tower is such that 29.8 pounds (133.2 cubic feet) of S03 are absorbed, 60% oleum product formation amounting to 77.5 poundsper minute. The gas leaving the top of the 60% oleum tower, at temperature of about 91 F., cntains 152.6 pounds (682.4 cubic feet) of S03 and has an S03 concentration of 20.3% by volume.

359.4 pounds of oleum from the stripper and 30.1 pounds of 99.3% sulfuric acid from the sulfuric acid production tower are introduced into the cooler of the 35% oleum production tower. vRate of cooling and of liquor circulation in the circulating system of the 35% oleum tower are regulated so that oleum of strength of about 33% and at a temperature of 110 F. is fed into the top of the 35% oleum tower. Gas and liquor contacting are such that 128.2 pounds (573.4

, 182.4 pOllnds (815.6 cubic feet) 0f S03, S03 00nl cubic feet) oi' S03 are absorbed, and 35% oleum production amounts to 517.6 pounds. The gas stream passing into the bottom of the sulfuric acid production tower is at temperature o1' about 110 F. and contains 24.4 pounds (109 cubic feet) vof S03, and has an S03 concentration of 3.9%-

by volume.

The liquor cooling and circulating system of the sulfuric acid tower is operated to produce 99.3% strength sulfuric acid, and when so proceeding 5.70 pounds per minute of diluting water are fed into the circulating system of the sulfuric acid production tower. All'of the 24.5pounds of S03 entering the-sulfuricacid tower is absorbed, and the stack tail gas is free of S03. Hence, in this example. the system is balanced, and no SO: is taken out of the system as sulfuric acid or oleum of intermediate strength.

In the past oleum up to about 40% strength has been made by direct absorption of converter S03 gas. The freezing point of 40% oleum is about 95 F.. and in order to prevent freezing it has been customary izo-introduce the' circulating oleum into-the top of the 40% oleum production tower as at 8B Fig. 2 of the'drawing at a temperature of about 105 F. At this tempera ture, the S03 content by volume of the vapor'in equilibrium with'the oleum in the top of the. tower is about 4.75%. Since converterv S03 gases may have an S03 content of say,7 to 11%, it has been feasible when using such gases to make as high as 40% oleum by cooling such SO: gases down to about 225 F. and introducing the cooler gases installations, the S03V gas is derived by catalytic oxidationl of the S02 gas produced by the roast ing of pyrites or pyrrhotite with the consequence lthat the S03 gas leaving the converter has an S0: concentration of usually less and rarely over about 8 V2% S03. Thus, in working with relatively low concentration gases it is necessary in the prior art practice for making say 40% oleum by direct 'absorption to employ relatively large size absorbing apparatus and correspondingly high capacity coolers and pumps in the oleum circulating systems. For example, in order to produce.40% oleum from 7% S03 gas it is necessary to provide gas cooling and absorption equipment capable of handling about four volumes of gas in order to absorb the amount of S0; present in one volume of gas. 0n the other hand, in order to produce 40% oleumfrom about a .10.7% S03 gas it is necessary to provide gas' cooling and absorbing equipment to handle only about 2.1 volumes of gas to absorb the S03 present in one volume of 10.7% SOagas. Thus,- it will be seen that by raising the S03 content of the gas from about 7 to about 10.7% the size of the equipment needed in the oleum production system is reduced almost 50%.

Another disadvantage encountered in practice, when producing oleum of say.40% strength from low concentration S03 gases. .is that 'it is extremely diiilcult to maintain aI balanced system without the uneconomical and almost lprohibitive use of a large number of absorbing towers. 'Ihe reason for this is that with a low concentration S03l gas, only a small portion of the S03 can be absorbed in.the 40% production tower,

and in order to clean all ofthe S03 out of th vgas stream the intermediate strength oleum tow` er and the sulfuric acid tower must be operated .strength oleums above, e. g. 40%, it should be understood that the invention is not thus limited but may be used also in the manufacture of oleumV of concentration of 40% or less. When so employed, the advantages provided are substantial reduction in size of the absorbing towers and associated oleum circulating circuits, operation of balanced systems, and further the utilization of weak gases (e. g. 4 or 5% S03) which lheretofore could not `be used for production of say 40% oleum by direct absorption.

Following Example 3 is illustrative of manufacture of 40% oleum in apparatus such as disclosed in Fig. 2 of the drawing.

Example 3 S03, 7.0% S03 by volume, is fed into the bottom of stripper il thru pipe vIii at temperature of about 440 F. and at a rate of 3000 cubic feet to S0; in a contact plant of the type in which a Gas containing 4.7 pounds (210 cubic feet) of v Il, 33.3 pounds heat exchanger is associated with each converter. Y

Hence, the 440 F. temperature of the gas in line I is substantially less than the temperature of the S03 gas discharged from the last converter. The oleum production tower 22 is operated so that the 20% oleum running out thru pipe 90 is at temperature a little above 180 F. 210.5 pounds of 20% oleum are drawn out from pipe 9| and fed`by pipe 92, at temperaturel of 180 F., into the top countercurrent flow of oleum and incoming S03 gas is such that a 5% oleum (177.3 pounds) at temperature of 270F. is discharged from stripper thru pipe 94 by means of which such partially stripped oleum is returned tocooler 33 and re,

of stripper in which' 10 introduced into the circulating system of the 20% oleum production tower 22. In stripper (148.7 cubic feet) of S03 are vaporized out of the 20% oleum fed in thru pipe 92, and the S03 gas leaving stripper thru pipe 20 95, at temperature of about 180 F., contains 80.3 pounds (358.7'cubic feet) of S03 at concenl tration of 11.4% by volume.

63.4 pounds perxminute of the oleum of tower 22 are run thru pipe 9| 32 of tower-2|. Liquor cooling and circulation are regulated so that oleum of about 39.5% strength is fed into the top of tower 2| thru pipe' 88 at' temperature of 120 F. Contact of gas and liquor in towerl 2| is such that 21.1 pounds (94.5 cubic feet) ofS03 are absorbed, 40% oleum product formation amounting to 84.5fpounds per minute. The gas leaving the top of the tower 2| thru line 91, at temperature make of 20% vof about 122v F.,contains 59.1 pounds (264.2

cubic feet) of S03 and has an S03 concentration of 8.7% by volume.v

177-.3 pounds of 5% 49.2 pounds of 99.3%

are introduced thru pipes sulfuric acid from tower 39 94 and 98 into cooler 33. Rate Iof cooling and of liquor circulation in the circulating-system of tower 22 are regulated vso that oleum of strength of about 18% and at a temperature of 120 F. is fed into the top of tower 22 thru pipe 99. Gas and liquor contacting are such that 46.4 pounds (212.2 cubic feet) of S03 are absorbed, and 20% oleum production amounts to 273.9 pounds. The gas stream passing thru -pipe |00 into the bottom of sulfuric acid production tower 39v is at temperature of about 120 F. and contains 11.7 pounds (52.2 cubic feet) of S03, and has an S03 concentration of 1.8% by volume.

The liquor cooling and circulating system oi sulfuric acid tower 39 is operated to produce 99.3% strength sulfuric acid, and when so p roceeding 37.2 pounds per minute of diluting 92.1% H2504 are fed thru pipe 5| into the circulating. system, of tower39. All of the 11.67 pounds of S03 entering thru-pipe |00 isabsorbed, and the stack tail gas is fr ee of S03. Hence, asin other examples, the system is balanced, and no S03 is taken out of the system as sulfuric acid or intermediate strength acid. About 37% of the total oleum production is derived from the incoming- 92.1% diluting acid. v

I claim:

1. In the manufacture of oleum in asystem including a catalytic S02 oxidation plant'forming ahot S03 exit gas, an oleum product production stage, at least one intermediate strength oleum production stage, and a sulfuric acid production. stage, in which system the S03 gas is passed thru Y said stages in the ordernamed` and absorbing liquor is ilowed in the reverse direction, the steps into cooler 25 oleum from stripper and l gas, of suiiicient IS03 from said low strength 0 oleum in. said absorption zone, and regulating into intimate comprising introducing into the sulfuric acid prohot S03 exit gas of said plant into said zone and direct counter-current contact with the dispersed oleum therein, regulating said contacting step .to effect desorption of S03 from said oleum by means of the sensible 1heat of said hot S03 gas while said oleum and gas are in direct contact, introducing the thereby resultant enriched S03 gas exiting the desorption zone into the oleum product production stage regulated to absorb S03 and form oleum product, and passing the S03 gas stream exiting the oleum production stage and containing residual S03 thru the intermediate strength oleum and sulfuric acid production stages.

2. In the manufacture of oleum in a system in-` cluding a catalytic S03 oxidation plant forming a hot S03 exit gas, an oleum productproduction stage, at least one intermediate strength oleum production stage, and a sulfuric acid production stage, in which system the S03 gas is passed thru said stages in the order named and absorbing liquor is flowed in the reverse direction, the steps4 comprising introducing into .the sulfuric acid production stage absorbing liquor of the group consisting of water and sulfuric acid, withdrawing intermediate strength oleum from an intermediate strength oleum production stage and dispersing the same in an S03 desorption zone, bringing the hot S03 exit gas of said plant into said zone and into intimate direct counter-current contact L regulating said contact, introducing the therebyv resultant en-` riched S03 gas exiting the desorption zone into the oleum product production stage regulated to' absorb S03 and form oleum product, passing the S03 gas streaml exiting the oleum production stage and containing residual S03 thru lthe intermediate strength oleum and sulfuric acid production stages, and regulating flow of S03 gas and of absorbing liquor thru said stages so that substantially all of the incoming S03 and any sulfuric acid absorbing liquor introduced into said stages aredischarged from the system as oleum product.

3. The process for making oleum of predeterthan can be made by digas of a catalytic oxidation plant which process comprises directly contacting said hot S03 gas in counter-current relation with dispersed oleum of strength lower than said predetermined strength,

regulating said contacting operation so as to effect release, by means of sensible heat of said S03 oleum to lform a resultant gas having an S03 concentration b`y volume in excess of the S03 concentration by volume in gas in equilibrium with oleum at the point of last contact of S03 gas with absorbing liquid in an absorbing zone in which oleum of saidrpredetermined strength fis produced, contacting said resultant S03 gas with said latter contacting operationto increase ,the S03 strength of such oleum up to said predeterminedstrength. v .i

BERNARD M. CARTER. 

